Time-Resolved Optical and X-Ray Spectroscopy of Rhenium Based Molecular Complexes

Abstract

The photocycle of rhenium carbonyl complexes, type facial-[Re(L)(CO)3(diimine)]n [L=halide / n=0, L=4-Ethyl-pyridine (Etpy)- Imidazole (ImH) /n=+1, diimine= bpy , 10- phenanthroline (phen), 4,4'-dimethyl-2,2'-bpy (dmb)] was studied using a set of ultrafast spectroscopic techniques: Fluorescence up-conversion, transient absorption and X-ray absorption spectroscopy. Relaxation of the initially excited singlet charge transfer (CT) b1A' state was investigated using fluorescence up-conversion (FlUC). The excited b1A' state undergoes intersystem crossing (ISC) simultaneously to two triplet states (a3A'' and b3A'') with a time constant τ1 in the 85-160 fs range that depends on the ligand L. An internal conversion process between the involved triplet states was found to occur with a solvent dependant lifetime τ2 of ∼500 fs in CH3CN and ∼1.4 ps in DMF. Femtosecond transient absorption measurements of the bpy-containing complexes revealed two processes. The first one, occurring in the same time range as the internal conversion measured using FlUC, also showed a solvent dependence. We attribute it to vibrational and electronic relaxation as well as solvation leading to population of the a3A'' state. The second strongly solvent-dependant process, is manifested mainly by a continuous rise of the bpy- absorption band at 370 nm. We assign it to reorganization within a supramolecular cluster consisting of the [Re(L)(CO)3(bpy)]n chromophore and several strongly interacting local solvent molecules, probably intercalated between the ligands. In this series of complexes, theory predicts that the ligand (L) is important in tuning the character of the lowest excited state. For an electron-rich L ligand, the time-dependent density functional theory (TD-DFT) calculations characterize the lowest CT state as a metal-ligand-to-ligand-charge-transfer (MLLCT) state which leads to the reduction of the diimine ligand. To confirm this prediction and to extract structural information about the excited state, we used X-ray absorption spectroscopy. We first characterized the ground state by static measurements at the Re L3 and the Br K edges of the Re(Br)(CO)3bpy complex. Picosecond X-ray absorption measurements of the excited complex revealed a charge transfer from both the Re and the Br sites, confirming thus the mixed excited state character. These results constitute the first evidence about charge transfer from a monoatomic ligand containing Re complex. The excited state structure was also extracted by an analysis of the XANES and EXAFS spectra at the Re L3 edge. It involved contraction of the Re-N and Re-Br bond distances, and elongation of the Re-C distance, supporting thus the MLLCT character. The structural analysis confirms the prediction from the TD-DFT calculations.